1. Field of the Invention
The invention relates to an exhaust gas recirculation device of an internal combustion engine, and a control method thereof.
2. Description of the Related Art
A known technology of reducing the amount of NOx contained in exhaust gas from an internal combustion engine is an EGR technology that recirculates a portion of exhaust gas to an intake passage. For example, Japanese Patent Application Publication No. JP-A-2004-150319 discloses an exhaust gas recirculation device of an internal combustion engine which includes a low-pressure EGR passage connecting an exhaust passage downstream of a turbine of a turbocharger and an intake passage upstream of a compressor of the turbocharger, and a high-pressure EGR passage connecting the exhaust passage upstream of the turbine and the intake passage downstream of the compressor, and which performs an EGR control by switching between the low-pressure EGR passage and the high-pressure EGR passage or using both EGR passages in accordance with the state of operation of the internal combustion engine.
In conjunction with an internal combustion engine equipped with a particulate filter (hereinafter, referred to as “filter”) that traps particulate matter (hereinafter, referred to as “PM”) contained in exhaust gas from the internal combustion engine, there is a known technology that removes the deposit of PM from the filter by oxidation in a forced fashion by raising the temperature of exhaust gas that flows into the filter (this process will be hereinafter referred to as “regeneration process”) if the amount of PM deposited on the filter is greater than or equal to a predetermined amount.
For example, Japanese Patent Application Publication No. JP-A-2003-343287 discloses a technology that performs the regeneration process of the filter through the temperature raise of exhaust gas from the internal combustion engine which is achieved by closing an exhaust throttle valve provided in an exhaust passage downstream of the filter so as to raise the back pressure in the exhaust passage upstream of the exhaust throttle valve and therefore increase the load of the internal combustion engine.
The exhaust gas recirculation device of an internal combustion engine equipped with an exhaust purification device, such as a filter or the like, generally has a construction in which the low-pressure EGR passage is connected to the exhaust passage downstream of the exhaust purification device, and an exhaust throttle valve is disposed in the exhaust passage downstream of the site of branching to the low-pressure EGR passage.
In this construction, for example, if the exhaust throttle valve is operated in the closing direction in order to execute the regeneration process of the filter, the pressure in the low-pressure EGR passage rises with the rise in the back pressure in the exhaust passage at the upstream side of the exhaust throttle valve. Therefore, there is a possibility that the adjusting accuracy of a flow amount regulator valve provided on the low-pressure EGR passage will decline and therefore the amount of EGR gas recirculated via the low-pressure EGR passage will become excessively large or small. Therefore, generally in the related-art exhaust gas recirculation device having a construction as described above, the recirculation of exhaust gas via the low-pressure EGR passage is stopped during the execution of the regeneration process of the filter.
In this case, however, during the regeneration process of the filter, there is a possibility that sufficient amount of EGR gas cannot be supplied to the internal combustion engine and therefore the amount of NOx emission will increase. Conversely, during a state of engine operation that requires large amount of EGR, there is also a possibility that the execution of the regeneration process of the filter will become impossible and therefore an excessive amount of PM will deposit on the filter and will degrade fuel economy.
This problem can possibly occur not only in the case where the exhaust throttle valve is closed at the time of the regeneration process of the filter, but also in, for example, the case where the exhaust throttle valve is closed at the time of quick warm-up of a catalyst, the case where the exhaust throttle valve is closed in order to operate an exhaust brake, etc.